The remainder of the paper is organized as follows. In Section 2,
for completeness and readability, we repeat Appendix A in [1]},
which discuss the solutions to a first-order ordinary differential
equaion. In Section 3, we show that, for travelling wave solutions,
Eq.(REF ) may be reduced to a first-order ordinary
differential equation involving two arbitrary integration constants
\(a\)  and \(b\) . We show that there are four distinct periodic solutions
corresponding to four different ranges of values of \(a\)  and
restricted ranges of values of \(b\) . A short conclusion is given in
Section 4.
